Gas turbine engines are divided into a number of ‘fire zones’, the different fire zones tending to operate at respectively different temperatures when the engine is functioning, and being separated by ‘fire walls’. The fire walls prevent any flammable fluid leakage between the various zones and help to prevent the spread of a fire from one zone to another.
Typically, the core region of the engine comprises one or two zones, normally referred to as Zones 2 and 3, and the region outside the fan case constitutes a separate zone, referred to as Zone 1. There is also a bypass zone in the region in which bypass air flows, between the core and the fan case.
Zone 3 is located generally radially inwards of Zone 1. However, typically Zone 3 is extended radially outwardly and downwardly into the general area of Zone 1, for a limited circumferential extent, in a lower region of the engine. This extended Zone 3 region forms a “bifurcation”, because bypass air is forced to pass around it, the air being directed by a splitter fairing.
It is necessary for pipes and harnesses to pass from the core region, for example from Zone 3, to the fan case region (Zone 1). At a base of the extended Zone 3 region, there is a ‘bifurcation disconnect panel’ through which all the pipes and harnesses extending from Zone 3 to Zone 1 pass. This panel forms a fire wall and allows the pipes and harnesses to be disconnected at the panel or removed from the panel for line replacement.
For engines with fan case mounted accessories, power for the accessories of the engine, for example the electrics, the hydraulic pump, the oil and fuel pumps, etc. is provided by a generator which is driven from the high pressure turbine shaft but which is mounted on the fan case in Zone 1. It is thus necessary to provide a drive means between the high pressure turbine drive shaft and the fan case, this drive means passing from Zone 3 to Zone 1. This is termed the radial drive. The radial drive must therefore pass through the bifurcation disconnect panel.
The above described prior art arrangement has certain disadvantages. The pipes and harnesses are usually required to cross the fire wall at a right-angle. There are also requirements for a minimum straight section before and after the disconnect region where the pipes and harnesses cross the panel, as well as a minimum bend radius. These constraints affect the pipe and harness routing, creating unnecessary bends. The resulting intricate design makes clashes more likely and harder to detect during design and makes modifications from an original design more difficult to accommodate and unnecessarily complex. The routing of the pipes and harnesses appears untidy and is therefore difficult to follow and likely to cause confusion and reduce accessibility for maintenance. It is also necessary to provide large numbers and various different types of brackets, lugs and clips to hold the pipes and harnesses. This increases the manufacturing costs and increases the removal/refit time.
A further disadvantage of the prior art arrangement relates to the routing of the radial drive. Currently, there is a D-seal between a radial drive shroud and the bifurcation disconnect panel. This causes various problems. Firstly, the fan excitation of the splitter fairing, as well as the relative movement between the core and the fan case, cause the D-seal to wear out quickly. For similar reasons, an O-ring seal provided between the radial drive shroud and the transfer gearbox has been known to fail in service, producing oil leakage. Secondly, the size of the D-seal support dictates how closely the splitter fairing can be wrapped around the radial drive. A larger splitter fairing is less aerodynamically efficient than a small one.